1. Field of the Invention
This invention relates in general to a method and apparatus to sense and report proper transmission line termination, and more particularly to a method and apparatus to sense and report the connection integrity of a differential ECL transmission line having proper parallel termination.
2. Description of Related Art
In recent high density Direct Access Storage Devices (DASD), the data to be written to a disk is transmitted from a data channel circuit to an arm electronics (AE) module through a differential transmission line. The channel chip typically drives this transmission line with a differential Positive Emitter Coupled Logic (PECL) driver circuit. The output stage of this circuit typically uses an actively biased emitter follower configuration.
Transmission lines must be terminated to minimize signal reflections which can cause power dissipation in the transmitted signal. Thus, signal termination circuitry provides a terminating impedance for the signal traveling down the transmission line. Ideally, this termination impedance should be connected to the end of the transmission line in such a way that any signal traveling down the line sees no change in impedance when it reaches the end of the signal path. That is, the termination impedance should be equal to the characteristic impedance of the transmission line (Z.sub.o). Maintaining this sort of impedance characteristic over the entire signal path will minimize signal reflections. This arrangement allows high bandwidth data to be transmitted with virtually no signal corruption.
One effective way to terminate a differential transmission line to minimize signal reflections is to place an impedance matching device between the two differential signal wires at the end of the signal path. This impedance value is typically designed to possess an impedance equal to the differential impedance of the transmission line itself.
Because this termination impedance effectively connects the two wires of the transmission line, it becomes difficult for the standard write safety circuits to always detect certain types of faults on this channel. For example, faults such as one wire open and both wires open are difficult to detect.
In applications where a matching termination is not required, such as applications using lower data rates, sub-optimal termination schemes have been employed in order to achieve other desirable features. A "single ended" termination approach has been used previously for two reasons: the proper bias for emitter followers having open emitter type outputs were provided and open interconnects could be easily detected.
FIG. 1 illustrates a circuit diagram 100 of a data channel 101 coupled to an AE module 103 via an interconnect 102, wherein two prior art termination methods 150, 160 are shown. In FIG. 1, a differential ECL transmission line 104 is shown that includes terminating impedance 133 at a receiver inputs 131 and 132. Impedances 140, 141 are coupled to ground on each conductor to provide the bias current to the emitter followers 112, 113 in the data channel circuits 101. As shown in FIG. 1, the data channel circuit modules include open emitter type outputs 112, 113. Connecting these impedances as shown in FIG. 1 makes detecting open interconnects possible. Since the impedance at the input of the AE module 103 would pull the voltage on an open line to ground, the AE module 103 would sense an invalid voltage level at one or both of its inputs 145, 146 and could post an error indicating an open interconnect condition.
As data rates increase, this sub-optimal termination scheme is no longer adequate and one must more accurately match the termination impedance to the transmission line. FIG. 1 shows a second termination impedance transmission line matching method 160. This termination impedance matching method 160 is accomplished by connecting an impedance 142 across the two conductors 131, 132 of the line as shown in FIG. 1.
While this impedance 142 can provide perfect signal termination, it makes it difficult to reliably detect an open conductor in the interconnect. This result can be understood by recognizing that the two conductors 131, 132 are now tied together by a relatively low impedance. If one conductor breaks, the signal on the driven input pin will simply couple through the termination to the other pin. This condition will cause both AE inputs 145, 146 to see nearly the same signal. The receiver will not be able to properly interpret this signal and will provide an unpredictable output to the rest of the circuitry in the AE module 103.
It can be seen that there is a need for a method for sensing and reporting the status of connection integrity for differential ECL transmission lines having proper parallel termination.
It can also be seen that there is a need for providing proper termination impedance needed to support increasingly higher data transmission speeds.